Xradia Inc. has recently developed the world's first X-ray tomographic microscope with a demonstrated 3D resolution of 70 nm for the semiconductor industry. It uses 5.4KeV X-rays from a rotating anode source. A complete data-set is collected and analyzed automatically in 8 hours, while a single 2D image is acquired in 10 minutes. The specific aim of this proposal is to upgrade this microscope so it can operate in phase contrast and dark-field mode for biomedical applications at 70 nm resolution in Phase I, and 30 nm resolution in Phase II. (Phase contrast is needed since biological specimens show very little amplitude contrast in the microscope). In Phase I a phase ring will I be designed, fabricated, incorporated into the microscope, and used to demonstrate 3D Zernike phase contrast imaging on yeast. In Phase II new optical elements will be developed to reach the 30nm resolution goal. The proposed microscope is unique in that it outperforms all other 3D X-ray microscopes in resolution and throughput, yet it uses a rotating anode X-ray source, rather than a synchrotron. It is designed to image single cells, cell cultures, or predetermined regions within tissue sections by "virtual sectioning". The specimens may be frozen hydrated, or fixed and labeled using conventional methods. These capabilities will make the instrument a n important complement to confocal microscopes (offering higher resolution) and electron microscopes (offering thick-specimen capability and simplified specimen preparation).